The Co-operative Bank has been downgraded by the credit rating agency Fitch. Photograph: Rui Vieira/PA

The Co-op Bank was dealt another blow on Thursday as it was downgraded by ratings agency Fitch, which warned there was a risk that its proposed £1.5bn fundraising might fail. Fitch also raised concerns that it could lose customers after the intervention of hedge funds, which are forcing the mutual Co-op Group of funeral homes, supermarkets and pharmacies to cede control of the bank. The Group's stake is to be cut to 30% from 100% – with bondholders led by US funds Aurelius and Silver Point taking the rest. Fitch said the new strategy could make it difficult to generate new capital, especially as the bank will not make a profit before 2014 at the earliest.

The downgrade, which coincided with the Queen's official opening of the group's new head office in Manchester, took the bank's rating to B from BB- and could be followed by a further downgrade if the restructuring plan fails. Bondholders must vote through the complex restructuring by 29 November. If they fail to back it, there is a possibility the bank could be taken under Bank of England control through a so-called resolution procedure. Fitch said there was "material risk to the success of the restructuring".

A Co-op Bank spokesperson said: "The Co-operative Bank notes the announcement from Fitch. The rating agency's response is anticipated and does not alter the Bank's confidence in the Recapitalisation Plan, which is focused on delivering the £1.5bn of capital necessary to put the Bank back on a stronger financial footing."

The Treasury select committee is to interview David Anderson, the former boss of the Co-op's financial services arm, next week as part of its review into why the deal to buy 631 branches from Lloyds Banking Group failed.